1. Field of the Invention
This invention relates to a method of forming a circuit mask and more particularly, to a method of forming the mask by means of a refreshable artwork master comprising a liquid crystal cell.
2. Discussion of the Prior Art
In the manufacture of printed circuits and integrated circuits (microcircuit patterns), increased and stringent demands have been made on the width and accuracy of the pattern delineated, e.g., conductor patterns of printed circuit boards. Conventional techniques of generating enlarged scale artwork by taping or cutting are not accurate enough for the present needs of both printed circuit board and microcircuit manufacture. Currently, very sophisticated systems are employed requiring a great number of various interconnecting patterns. Recently, computerized, electromechanically driven artwork generators have been employed which eliminate most accuracy problems. Usually masters are prepared by such computerized pattern generators, such as emulsion on glass masters, which are permanent and which have to be stored and indexed and which are not readily adaptable to pattern modification. This is a problem since the generation of printed circuit patterns, as well as microcircuit patterns, requires adaptation to changing sizes, configurations and degrees of complexity. There are no computerized (or other) pattern generator systems presently capable of rapid response to changes in size, configuration and/or degrees of complexity whereby a refreshable master is generated. By a refreshable master is meant a master which can be rapidly altered, as by addition thereto or subtraction therefrom, to yield a new and different master having a different pattern contained therein.
Recently, liquid crystal compositions and apparatus have received wide interest in the field of display systems, such as in advertisement devices, temperature indicating devices and so forth. However, it had not heretofore been thought that such compositions and apparatus, such as used for graphics or alpha-numeric displays, could be employed for accurate projection display, such as that required for printed and microcircuit pattern generation, because of the poor contrast and resolution of the patterns projected thereby.
A number of prior art arrangements include means for "writing" on a liquid crystal cell by inducing light scattering regions in otherwise clear liquid crystals. In particular, in a well-known arrangement, the cell includes a photoconductive layer, the impedance of which changes in relation to the intensity of light incident on it. During the writing mode, an electric potential is maintained across the liquid crystal substance and photoconductive layer of the cell. At the point of incidence of the light beam on the cell, the impedance of the photoconductive layer, and hence the voltage across it, decreases, thereby increasing the voltage drop across the liquid crystal. The resulting increase in voltage across the liquid crystal produces scattering areas within the liquid crystal. Such an arrangement is described, for example, in an article entitled, "Reversible Ultraviolet Imaging with Liquid Crystals" by J. D. Margerum et al., appearing in Applied Physics Letters, Vol. 17, pages 51-53, July 15, 1970. See also G. H. Conners et al., U.S. Pat. No. 3,592,527, issued July 13, 1971.
In addition, it is known to use a laser beam as the light source to alter the resistance characteristics of the photoconductive layer. The typical configuration in which such cells, containing a photoconductive layer, are used further includes a source of projection light for projecting an image on a display surface.
However, it is a characteristic of the mentioned photoconductive materials that they alter the optical characteristics of the system. In particular, these materials typically absorb a range of frequencies of the projection light shone on them, thus causing a degenerative effect on the projected image. Special precautions often involving expensive and intricate apparatus must therefore be used to avoid these degenerative effects. The precautionary measures often result in inefficient use of the projection light yielding an image having poor resolution.
The prior art liquid crystal cells are generally of either the transmissive type, in which scattered projection light passes through the entire cell, or the reflection type, in which the projection light is reflected from an internal surface. The images projected in the prior art arrangements exhibiting the transmissive mode are typically characterized by dark writing on an orange or, at best, pale yellow background, depending on the substances used. Contrast is even more difficult to obtain when the reflection mode is used primarily because of spurious reflection at the substrate surfaces. Further, it is difficult to fabricate reflecting layers which reflect a significant amount of projection light while, at the same time, providing efficient electrical conduction in a direction perpendicular to the plane of the reflecting layer, or, stated differently, minimizing undesirable current flow in the plane of the reflecting layer.
As discussed above, printed circuit and microelectronic circuit pattern delineation require high-feature density. The feature density is usually specified in terms of smallest linewidth divided into largest pattern dimension. This number varies between 5000 and 15000 for thin film, printed wiring board, multilayer board and integrated circuit patterns. Conventional display systems, e.g., graphics displays, alpha-numeric displays, while quite satisfactory for those applications cannot meet the space-bandwidth product number, given above, which is required for circuit patterns, and have never, heretofore, been employed for forming a circuit mask or for forming circuit patterns in any manner.
A pattern generator having a refreshable artwork master comprising a liquid crystal cell from which an image can be projected with high resolution and contrast is therefore desired.